Linearly vibrating vibration gyroscopes are generally known. In these rotation rate sensors, parts of the sensor structure are actively set into vibration (primary vibration) in one direction, i.e. in a first axis (x axis), which is oriented parallel to a substrate surface. At an outer rotation rate about a singular sensitive axis, Coriolis forces are exerted on the vibrating parts. These Coriolis forces, which vary periodically with the frequency of the primary vibration, give rise to vibrations of parts of the sensor structure (secondary vibration) that are also parallel to the substrate surface in a second direction or second axis (y axis) which is oriented perpendicular to the x axis. Means of detection are mounted on the sensor structure which detect the secondary vibration (Coriolis measuring effect).
In the lay-out of the rotation rate sensor, as described above, by design (choice of suitable symmetries) a singular cartesion coordinate system, K=(x,y) is specified for the primary and the secondary vibration within the plane of the substrate. The mass distributions and the spring distributions are laid out so that the main axis system of the mass tensors and spring stiffness tensors for the primary and secondary vibrations coincide exactly with K. In addition, in the implementation of the means of detection, care is taken that no signals are created at the means of detection for the Coriolis effect by the operation of the sensors in the primary vibration (without external rotation rate). For this purpose, the means of detection are designed so that their singular coordinate system KD also coincides with the coordinate system of the mechanics K, i.e. it is also true that KD=(x,y). Consequently, in the case of such ideal vibration rate sensors, there is not created a bridging of the primary vibration to the detection device for the Coriolis effect. Such a bridging is called a quadrature. Thus, quadrature signals are signals to the means of detection for the Coriolis effect, which are present also without a relative motion of the sensor with respect to an external inertial system, the sensor being operated in its primary vibration.
The quadrature leads to periodic signals, modulated with the frequency of the primary vibration, to the means of detection for the Coriolis effect.
The reason for the appearance of quadtature signals is that the coordinate system of the sensor element mechanics K=(x,y) does not coincide with the coordinate system of the means of detection KD=(x′,y′) but both systems are slightly rotated with respect to each other by an angle theta.
Typical causes for this rotation, which is generally slight, are, for example, asymmetries in the sensor structure by reason of imperfections in the manufacturing process. These are able to make themselves known by asymmetrical mass distributions or asymmetrical spring constants. As a result of this, the main axis systems of the mass tensors and the spring constant tensors no longer coincide with KD.
The appearance of quadrature is not specific for the silicon technology used for the rotation rate sensors described here, having a sensor structure made of epitactically grown polysilicon. Even in rotation rate sensors made of monocrystalline silicon material or with monocrystalline quartz, quadrature signals appear as a result of imperfections in the manufacturing process.
Another interpretation of the quadrature signals, important to an understanding of the present invention, is based on an observation with respect to interference theory: For a small twisting of the coordinate systems one may initially regard the directions of the main axis systems as interference free (K=KD). In this representation, the quadrature is described as s slight coupling of the two essential vibration modes (primary and secondary vibrations). In this representation, during the vibration of the sensor structure in primary mode, the quadrature leads to an inducement of the secondary vibrations, even without external rotation rate. This motion becomes visible as an interference signal at the means of detection for the Coriolis effect.
According to the present invention, based on the well-directed effect of time-wise periodically varying forces, a reduction or avoidance of quadrature signals is achieved. For this, electrostatic forces that vary in time (dynamic) are exerted on the sensor structure by electrode structures applied at suitable parts of the sensor structure and by the purposeful application of external electrical dc voltages. It is achieved particularly by the suitable form of the electrode structures (quadrature compensation structures) that, during the primary vibration of the sensor structures, forces varying in time act upon suitable parts within the sensor structure. These forces are oriented in such a way that they induce secondary vibrations, and may consequently be detected at the means of detection of the Coriolis effect. Because of the height of the electrical voltage, the magnitude of these signals may be varied until they exactly compensate the quadrature signals present in the sensor element because of imperfections. Consequently, the present invention represents a dynamic method for quadrature compensation.
The effect of the quadrature compensation is based, in the method according to the present invention, on a purposefully undertaken asymmetry within the mechanical sensor structure.
Quadrature interference signals in rotation rate sensors as a result of manufacturing imperfections are known, and are encountered in rotation rate sensors of the most varied technologies. In this context, according to the related art, various different methods are known for the reduction of these interference signals.
A first method, according to the related art, for suppressing quadrature signals makes use of the different phase position of rotation rate signals and quadrature signals. The Coriolis force is proportional to the speed of the primary vibration, whereas the quadrature is proportional to the excursion of the primary vibration. Consequently, there is a phase shift of 90° between the rotation rate signal and the quadrature signal. At the means of detection, quadrature signals and rotation rate signals are detected as signals amplitude-modulated with the frequency of the primary vibration. By the method of synchronous demodulation, as described, for example, in German Published Patent Application No. 197 26 006 and U.S. Pat. No. 5,672,949, the signals may first of all be demodulated again into the baseband. In addition, by a suitable choice of the phase position of the reference signal for the demodulation, the quadrature signal may be suppressed.
In this method, the quadrature signal is not influenced in the sensor element itself. Furthermore, the quadrature signal also has to pass through the primary signal conversion paths in the means of detection, and it is able to be electronically suppressed only relatively late in the signal path. In the case of large quadrature signals compared to the rotation rate measuring range, this means drastically increased requirements on the dynamic range of the first signal conversion stages, and often leads to increased sensor noise.
A second method according to the related art, for reducing quadrature signals, is the physical balancing of the mechanical sensor structures. Here, in contrast to the first method, the cause of the quadrature is directly rectified by reworking the sensor element, so that no quadrature signals appear at the means of detection. In the case of precision rotation rate sensors, this is achieved actively by iterative mechanical material surface removal at different places in the sensor element. Using this method, the principal axis system of the mass or spring constant tensors for the primary and secondary vibrations are modified so that the twisting of the coordinate system of the sensor element mechanics K with respect to the coordinate system of the means of detection KD, which is present at first, is reversed. In the case of rotation rate sensors made of monocrystalline quartz material, a surface removal of material is undertaken partially by laser trimming at singular locations in the sensor element. Here too, the mass tensor or spring constant tensor is modified so that, at the end, the twisting of K with respect to KD is essentially reversed. Even in the case of micromechanical rotation rate sensors made of monocrystalline silicon, laser trimming is used on mass structures (e.g. VSG or CRS-03 from Silicon Sensing Systems Ltd.). Furthermore, for general tuning fork rotation rate sensors, laser trimming at singular spring structures within the sensor structure is generally known. Using this method, in the operation of the sensor elements in primary vibration, the principal axis system of the spring constant tensor is able to be modified until K and KD coincide, and thus the quadrature signal is eliminated. The methods described here eliminate the quadrature in the sensor element itself, and are therefore superior to the first method, with respect to sensor performance. However, the balancing (procedure) represents a costly and often iterative as well as tedious process, and thereby a very cost-intensive process.
According to a further generally known method according to the related art, an electronic quadrature compensation is carried out in capacitive micromechanical rotation rate sensors. By doing this, the suppression of the quadrature signal is accomplished by the targeted injection of an electrical signal into the electronic transducer unit at the means of detection for the Coriolis effect. For this, the magnitude of the signal is selected so that it exactly compensates for the signal generated by the quadrature at the means of detection. In this method too, (analogous to the first method according to the related art), the mechanical cause for the quadrature signal itself is not eliminated. However, in contrast to the first method, in this case the quadrature signal is suppressed even before the primary signal conversion. This is able to reduce the requirements on dynamic range and noise of the primary signal conversion. However, a serious disadvantage of the method described is that it is suitable only for a very special design of the sensor evaluation electronics. This evaluation method (baseband evaluation), however, has serious disadvantages conditioned on principle (electrical distortion, etc), and therefore cannot be used in rotation rate sensors described in the present invention.
In U.S. Pat. No. 6,067,858, a further method according to the related art for electronic quadrature compensation in capacitive micromechanical rotation rate sensors is discussed. Between movable comb fingers and fixed electrodes, different electrical potentials are applied.